Method of optimizing the operation of a xylene separation unit using simulated countercurrent

ABSTRACT

Method of optimizing the operation of a unit intended for separation of the components of a feed (xylenes) by simulated countercurrent in hybrid operating mode. 
     The method allows to minimize the solvent ratio and to maximize the capacity of the separation unit while keeping product specifications such as purity and yield constant. It has been verified that these two objectives cannot be reached simultaneously and it is recommended to operate with a minimum solvent ratio while guaranteeing a high capacity compatible with stable operation of the separation unit. These optimization objectives are reached while keeping good stability around the optimum point thus defined, by using a known operation control process such as the one described in patent EP-875,268 for example.

REFERENCE TO RELATED APPLICATIONS

This application is a 371 of PCT/FR03/02327 filed Jul. 23, 2003.

FIELD OF THE INVENTION

The present invention relates to a method of optimizing operation of asimulated countercurrent xylenes separation method under hybridoperating conditions.

BACKGROUND OF THE INVENTION

Chromatography-based separation or fractionation methods are most oftenimplemented in a separation system comprising (FIG. 1) a series ofcolumns or column fractions interconnected in series, forming a closedloop. A porous solid of predetermined grain size constitutes thestationary phase. The mixture to be separated is fed into the column,then displaced by means of a carrier fluid or desorbent (EL) and thevarious constituents flow out successively according to whether they areretained more or less greatly by the stationary phase. Injection pointsfor the mixture or feed F containing all of the constituents to beseparated and the solvent or desorbent EL, and extraction points for anextract Ex containing the product to be upgraded, diluted in solvent,and for a raffinate Rf containing all the other constituents aredistributed along this loop. These points delimit various zones (Z1 toZ4 for example). An identical liquid stream flows through all thecolumns or column fractions of a zone. A pump P is arranged somewhere inthe loop to provide circulation of the fluid in the direction shown inthe diagram.

In a real countercurrent separation system, a fixed and constantconcentration profile develops where the positions of the injection andextraction points remain fixed. Adsorbent solid 3 and liquid 2 move in acountercurrent flow. A solid entrainment system and recycle pump P, botharranged at the junction of zones Z1 and Z4, respectively allow to sendback the solid from the base to the top and, conversely, the liquid fromthe top to the base.

Systems known as simulated moving bed systems allow to overcome a majordifficulty inherent in true moving bed methods, which consists inproperly circulating the solid phase without creating attrition andwithout considerably increasing the bed porosity in relation to theporosity of a fixed bed. To simulate its displacement, the solid isplaced in a certain number n of fixed beds (generally 4≦n≦24) arrangedin series and it is the concentration profile that is displaced at asubstantially uniform velocity all around a closed loop. In practice,successive switching of the injection and extraction points is obtainedby means of a rotary valve or more simply of a series of suitablycontrolled on-off valves. This circular switching, carried out at eachperiod, of the different incoming-outgoing liquid flows in a givendirection amounts to simulating displacement of the solid adsorbent inthe opposite direction.

The separation systems used for xylenes separation most often consist offour main zones. There are also systems with five zones where part ofthe extract separated from the solvent is reinjected between extractdraw-off and feed injection. Others can also have five to seven zoneswhere secondary fluids allow to rinse lines carrying successivelyseveral fluids so as to prevent contaminations.

In the text hereunder, the following variables are defined as:

-   -   controlled variables: variables that have to be constantly close        to a previously determined set value and which show the smooth        running of the process. It can be, for example, the purity of        the constituents of an extract, the yield of the separation unit        for a given constituent, etc.    -   operating variables: variables that can be modified by the        operator, such as the flow rates or the valve switch period        allowing to simulate displacement of the beds, etc.    -   control variables: variables that act mainly on a single zone,        for example on the part of the concentration profile contained        in a zone. These control variables are determined by the control        algorithm and are translated into operating variables.

It can be reminded that the goal of an advanced control system appliedto a process is to calculate a control law (all of the values of theoperating variables in time) so as to:

-   -   control operation, i.e. calculate a control law that can ensure        the transition between two distinct values of one or more a        priori selected controlled variables, and    -   regulate operation, i.e. calculate a control law allowing best        compensation (in advance or at least asymptotically) of all the        outside disturbances acting on the process so that the a priori        selected controlled variables keep a quasi-constant value.

In the case of a simulated countercurrent separation unit, regulationcan also compensate for disturbances due to an evolution with time ofthe thermodynamic and geometric parameters of the adsorbent (of coursefor a limited deterioration of the adsorbent properties).

These objectives are reached with the automatic control process based oneither a “black box” type technique, or on a more controlled approachallowed by non-linear modelling of the separation process.

Patent EP-875,268 (U.S. Pat. No. 5,902,486) filed by the applicantdescribes a method intended for automatic control of a simulated movingbed separation system for constituents of a mixture of circulatingfluids, notably aromatic hydrocarbons, which can have notable flow rateor feed quality variations. Control of the process (of non-linearmultivariable type carried out from a knowledge or linear model in theneighbourhood of a given working point, performed from input/outputrepresentation models) is carried out with a certain number of variablemeasurements at a plurality of measuring points along the loop(concentrations and flow rates for example) and of characteristicmeasurements of the fluids injected and extracted. Ratios respectivelyindicative of the ratio, in each zone, between the fluid flow rates andthe simulated adsorbent substance flow rates are determined from currentcontrolled variable values (constituents purity, yield of the system,etc.) depending on the measured variables. Values to be given to theoperating variables to bring or bring back the controlled variables todetermined set values are determined from these ratios. If fourindependent control variables are available for example, the four ratiosin each zone, four controlled variables have to be determined.

The control process comprises a calculating algorithm which determinesthe ratios from the measurements obtained, which are necessary forcalculation of the controlled variables. This calculation can be carriedout in two completely different ways: either using a non-linear physicalmodel of the true moving bed separation unit, or using a combination ofmonovariable linear models, each representing the behaviour of an output(a controlled variable) in relation to an input (a control variable),knowing that combination of these linear models is often referred to as“black box” by specialists. Determination of these simple models isperformed from a set of experimental measurements obtained on theprocess working in a state close to its planned stable state.

In its developed xylenes separation version, the process is used topurify the paraxylene present in feeds containing mostly xylenes, butalso C9 aromatics and paraffins in limited amounts. It is available intwo versions: the standalone version, which allows to reach a purityabove 99.80%, and the hybrid version which is dimensioned to reach apurity of the order of 95.00%. The latter version of the process,described for example in patent EP-531,191, is marketed with addition ofa crystallization process allowing to reach the desired high purity.Units working in hybrid mode consist of at least 12 columns, whereasthere are at least 24 columns for the standalone mode.

Whether a non-linear automatic control process or a black box typeprocess, the goal is to calculate, from measurement of theconcentrations of certain constituents necessary for calculation ofcontrolled variables, ratios (Rk) respectively indicative of the ratio,in each zone, between fluid flow rates (Qk) and the simulated flow rateof adsorbent material (Qs) so as to bring or to bring back thecontrolled variables to determined set values. In a second stage, thevalues of the ratios thus determined will be converted to operatingvariables applied to the process by means of conversion formulas.

The control process thus allows the separation unit to be brought to aworking point where the following four parameters are brought tospecified values:

1. The purity of the paraxylene in the extract defined as follows:

${{Purity} = \frac{{Px}^{extract}}{{Px}^{extract} + {IMP}^{\;{extract}}}},{where}$

-   Px^(extract) is the paraxylene concentration in the extract, and-   Imp^(extract) represents all the impurities in the extract.

Determination of this controlled variable requires online measurement inthe extract, on the one hand, of the paraxylene concentration and, onthe other hand, of all of the other constituents;

2. The paraxylene yield of the unit defined as follows:

${{Yield} = {1 - \frac{Q_{raffinate}{Px}^{raffinate}}{{Q_{raffinate}{Px}^{raffinate}} + {Q_{extract}{Px}^{extract}}}}},{where}$

-   Px^(extract) and Px^(raffinate) respectively represent the    paraxylene concentration in the extract and in the raffinate, and-   Q_(raffinate) and Q_(extract) respectively represent the raffinate    and extract flow rates.

Determination of this controlled variable requires online measurement ofthe paraxylene concentration in the extract and in the raffinate, andmeasurement of the extract and raffinate flow rates.

3. The amount of ethylbenzene Eb_(extract) in the extract.

Determination of this controlled variable requires the same onlinemeasurement as developed for point 1.

4. The amount of paraxylene Px_(zone1) at a point of zone 1.

Determination of this controlled variable requires development of aspecific measuring point in zone 1, i.e. between solvent injection andextract draw-off.

If the first two controlled variables clearly correspond to productionobjectives, the last two are directly linked with the object of thepresent invention, i.e. optimization of the unit operation.

Control of the separation unit requires concentration measurements atthree distinct points of the loop. These measurements are carried out bymeans of chromatography or Raman spectrometry, as described in patentFR-2,699,917 (U.S. Pat. No. 5,569,808) filed by the applicant. Onlinecalculation of the purity and of the yield and measurement of the amountof ethylbenzene in the extract requires two measurements which providethe concentrations of the different constituents in the extract and inthe raffinate. The last output Px_(zone1) requires a measuring point inzone Z1. The length of an analysis ranges between some seconds (Ramanspectrometry) and 20 minutes (chromatography). Considering the responsetime of the unit (between 4 and 8 hours), the quality of the processcontrol is not affected by the analysis time if it remains less than onehour.

Conversion of the control variables (ratios Rk) to “conventional”operating variables is always possible, apart from the real physicalapplication constraints linked with the dimensioning of the process andits equipment, because there is a one to one relation between them, anecessary condition for making the separation system perfectlycontrollable.

It is well-known that operation of a simulated countercurrent separationsystem is nearly identical to that of a true moving bed system if, forthe latter, the flows circulating countercurrent to the main liquid flowmeet the equivalence relations described in the aforementioned patentEP-875,268.

The control variables (ratios Rk) are determined in relation to theseequivalences as the dimensionless ratios between the main liquid flowrates in each zone and the solid flow rate which is constant in thewhole separation unit:

$R_{k} = {\frac{Q_{k}}{Q_{s}}.}$

Selection of these ratios follows from writing of the material balanceequations of the model of a true moving bed separation unit in thestationary state in a column portion that is discretized. The number ofratios is equal to the number of zones that make up the unit, each zonebeing characterized by a main liquid flow rate distinct from thecontiguous zones.

SUMMARY OF THE INVENTION

The method according to the invention allows to optimize operation of aunit intended for separation of the constituents of a feed andcomprising a separation loop consisting of the interconnection of aseries of beds containing a solid adsorbent material forming severalzones delimited by feed and solvent injection points and extractionpoints for discharge, out of the loop, of an extract containing a firstconstituent of the feed, and of a raffinate, injection points andextraction points switching means allowing to simulate countercurrentdisplacement of the beds and means for measuring operating variables.

It comprises using a control algorithm for bringing the separation unitto a working point where the purity of the first constituent in theextract (such as paraxylene for example) and the yield of the separationunit as regards production of this first constituent are brought tospecified values.

According to a first implementation mode, for a given value of theconcentration, in the extract, of a second constituent of the feed (suchas ethylbenzene for example), the set value of the concentration of thefirst constituent is adjusted in a zone located between the solventinjection point and the extract extraction point so as to minimize theproportion of solvent in relation to the feed.

Adjustment of the set concentration value is advantageously obtained bymeans of a monovariable optimizer.

Adjustment of the set concentration value of the second constituent inthe extract is preferably achieved to maximize the capacity of theseparation unit within the working stability limits of said unit.

According to another implementation mode, the set value of theconcentration, in the extract, of the second constituent of the feed isadjusted to obtain maximization of the capacity of the separation unitwithin the working stability limits of said unit.

The set concentration value, in the extract, of the second constituentpreferably ranges between 0.02% and 2%.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the method according to the inventionwill be clear from reading the description hereafter of an embodimentgiven by way of non limitative example, with reference to theaccompanying drawings wherein:

FIG. 1 diagrammatically shows a separation unit with four zones havingintercalated injection and extraction points,

FIGS. 2 a, 2 b respectively show the variation of the solvent ratio(S/F) and of the mean recycle flow rate (MR) as a function ofPx_(zone 1) and Eb_(extract), and

FIG. 3 shows the concentration profiles (C %) of Px, Eb, Mox along theseparation loop at the optimum working points in the sense of (S/F), atconstant purity and yield.

DETAILED DESCRIPTION

The method according to the invention allows to optimize operation of axylenes separation unit in hybrid mode with four zones, first brought,by applying the control process which is the object of theaforementioned patent EP-875,268, to a working point where twocontrolled variables characterizing directly the quality and theproduction of the product, i.e. the purity of the paraxylene in theextract and the paraxylene yield defined above, are brought to specifiedvalues.

Optimization is conducted using the control algorithm described therein.In order to optimize operation, since we have four independent controlvariables (the four ratios of each zone) and since two other controlledvariables have to be determined, we act upon:

-   -   the ethylbenzene concentration in the extract, Eb_(extract),        that can be advantageously determined using the measuring means        used for measuring the purity, and    -   the paraxylene concentration at a given point of zone Z1        (defined as the zone contained between solvent injection and        extract draw-off), Px_(zone1), which can be determined by        measuring the paraxylene concentration at a given point of zone        Z1.

The ethylbenzene concentration in the extract allows to characterize theposition of the ethylbenzene profile in zone Z2. The paraxyleneconcentration at a given point of zone Z1 is determined to control theparaxylene flow downstream from the solvent injection point.

The last two controlled variables are used to maximize or to minimize afunctional defined as a function of a priori set production objectives(economic cost for example) for this type of separation unit, such as,for example:

1. Minimization of the solvent ratio defined as the ratio between thesolvent flow rate and the feed flow rate.

2. Maximization of the feed flow rate.

3. Minimization of the mean recycle flow rate defined as follows:

${Q_{mean} = {\frac{1}{{Nb}_{column}}{\sum\limits_{i = 1}^{NbZone}{l_{i}Q_{i}}}}},{where}$

-   Nb_(Zone) represents the number of zones of the unit-   Nb_(column) represents the total number of columns-   l_(i) represents the number of columns in each zone-   Q_(i) represents the liquid flow rate in each zone.

Only points 1 and 2 are going to be dealt with in the descriptionhereafter, because optimization of point 3 is equivalent to that ofpoint 2.

Points 2 and 3 are optimized together because they are connected by thehomogeneity of the system of equations relating the ratios and theoperating variables. Any flow rate increase can be compensated by anequivalent increase in the other flow rates and a decrease in the sameproportion of the valve switching period. Maximization of the feed flowrate can therefore be directly associated with minimization of therecycle flow rate in the sense where the lower the recycle flow rate,the greater the margin for increasing the feed flow rate. These “ideal”considerations are limited in practice by hydrodynamics such as, forexample, the increase in the axial dispersion which varies quadraticallywith the velocity of the fluid in the outside porosity.

Point 1 relates to the excess consumption of solvent whose distillationcost is high. Optimization of the separation unit, in the sense ofpoints 1 and 2, is obtained by judicious adjustment of the outputsPx_(zone1) and Eb_(extract).

Simulation Results

We show with the simulation that these optimization objectives are notindependent and that there is an absolute minimum for the solvent ratiodepending both on Px_(zone1) and Eb_(extract). We will see, with theexperimental results, that we recommend operation under optimumconditions (to guarantee unit stability) however allowing to obtain asignificant solvent ratio and capacity gain.

The results obtained with simulation are summarized in the graphs ofFIG. 2.

The curves represent the variation of the solvent ratio (S/F) and of themean recycle flow rate as defined above, as a function of the value ofPx_(zone1) (abscissa) for a given Eb_(extract) value, at constantpurity, constant yield and constant feed flow rate.

The variations of (S/F) are strictly concave both in relation toPx_(zone1) (obvious from the curves) and to Eb_(extract) since the curve(Eb_(extract)=1.5%) is above curves (Eb_(extract)=0.5%) and(Eb_(extract)=0.9%) and below curve (Eb_(extract)=0.35%). There istherefore an absolute minimum for (S/F) whose value is not representedin these curves.

The variation of the mean recycle flow rate (MR) in relation toPx_(zone1) and Eb_(extract) is strictly increasing monotonous. The valueof the mean recycle flow rate (MR) decreases when Eb_(extract) increasesand increases when the value of Px_(zone1) increases, all the otherspecifications being constant.

The results presented above show that simultaneous optimization ofpoints 1 and 2 (i.e. minimization of S/F and maximization of the feedflow rate) is not possible. There is an absolute minimum for (S/F) whichdoes not correspond to the possible minimum likely to be reached by themean recycle flow rate.

Experimental Results

The experimental results obtained in the pilot unit confirm thetendencies shown by the simulation.

In the example hereafter, only the effect of Px_(zone1) is presentedbecause the influence of Eb_(extract) on the unit operation is clearlymore evident and therefore requires no specific experiments.

Comparison of the following 2 stationary points:

Stationary point No. 1 Stationary point No. 2 Purity = 95% Purity = 95%Yield = 96% Yield = 96% Eb_(extract) = 0.06% Eb_(extract) = 0.06%Px_(zone1) = 4% Px_(zone1) = 1.8% Q_(feed) = 68 cc/min Q_(feed) = 78cc/min Q_(recycle) = 379 cc/min Q_(recycle) = 379 cc/min S/F = 1.15 S/F= 1.1shows that, for the same purity and yield specifications, it is possibleto pass 10 cc/min feed more, to obtain a slightly lower solvent ratioand to keep the same recycle flow rate by changing only the set value ofPx_(zone1).

Characteristics of the Optimum Points Obtained

In order to compare the different optimum points (in the sense of theminimization of (S/F)) obtained above by simulation, we trace on thefollowing graph all of the concentration profiles C along the columns ofthe separation loop for three significant values of Eb_(extract).

The vertical lines of the graph of FIG. 3 respectively represent, fromleft to right: extract draw-off point Vex, feed injection point V_(F)and raffinate draw-off point V_(RAF).

The profiles go down when the mean recycle profile decreases. Close toextract draw-off point Vex, the various profiles are quite distinctbecause the proportions between the impurities in the extract varysubstantially since one of them (ethylbenzene) is the parametercharacterizing these simulations. In zone Z3, contained between feedinjection V_(F) and raffinate draw-off V_(RAF), the profiles of thevarious simulations are rather “close”. Their shape clearlycharacterizes the optimum operating modes of this separation unit, i.e.raffinate extraction is always at the base of the paraxylene profile inzone Z3. The differences obtained for the paraxylene concentration valueat the level of the raffinate draw-off can be explained by a constantyield value for all the simulations. The small variations of the Pxconcentration value compensate for the variations of the raffinate flowrate value specific to each simulation.

Separation Unit Optimization and Operation

After examining the results obtained by simulation, completed by theexperimental results, we can set out the following rules for optimumoperation of the separation unit in hybrid mode in the sense ofminimization of ratio (S/F) and of maximization of the capacity, i.e.adjustment of the separation unit so that it can potentially process amaximum amount of feed. Application of this optimization strategy ispossible in practice only by means of a control algorithm such as, forexample, the algorithm presented in the aforementioned patentEP-531,191.

Judicious selection of set values Px_(zone1) and Eb_(extract) allows toreach the separation unit operation optimum in the sense of minimizationof (S/F) for a given purity and yield.

Considering that we know, from the surveys carried out by simulation,that the curves are always strictly concave (FIG. 2) and that, byvarying Px_(zone1), we can move along such curves, and because of theconstraints imposed on set value Eb_(extract) to guarantee maximizationof the capacity while guaranteeing unit stability, a very simple simplextype monovariable optimizer, well-known to specialists, can be used foriterative automatic online search for the optimum set value ofPx_(zone1). Such an optimizer is practical because it can work withouthaving to calculate numerical gradients. The advantage is that it limitsthe number of evaluations of the cost function which, in the case of thepresent method, correspond to as many working points of the separationunit potentially outside the optimum working point.

Within the context of this application, optimization will be achievedwith generation of triangles in the plane, each vertex of the trianglebeing a potential solution. At each stage of the search for the optimum,a new point, in the current triangle, or close thereto, will beproduced. The value of the function at the new point is compared withthe values of the function at the vertices of the simplex, and usuallyone of the vertices is replaced by the new point, thus giving a newsimplex and a better estimation of the cost function. This stage isrepeated until the diameter of the simplex is smaller than the toleranceselected.

Judicious selection of set value Eb_(extract) allows to guaranteemaximization of the unit capacity, but with the following two comments:

-   1. The optimum value of this set value cannot be reached because, in    this case, the operating conditions reached by the unit are not    stable. In fact, a very slight decrease in the recycle flow rate    generates an increasing amount of ethylbenzene accumulated in the    column, which makes the separation unit difficult to operate because    the dynamics of increase of the amount of ethylbenzene in the    extract is much faster than the decrease dynamics.-   2. The optimum set value for Eb_(extract) in terms of capacity does    not coincide with the value sought for optimizing (S/F), which is    smaller.

Considering the two comments above, it is preferable to favour theseparation unit stability and to select, in this context, a set valueEb_(extract) guaranteeing maximization of the capacity, without tryingto reach the possible optimum. In this configuration, optimization of(S/F) amounts to the optimization described above.

Considering the experimental and simulation results, the value selectedfor Eb_(extract) directly depends on the performance of the controlsystem of the separation unit and on the purity value selected.

The steepness of the profile (all the profiles of the varioussimulations are similar) in zone Z3 and the specific position of theextraction point for raffinate Raf at the base of this front show a highsensitivity of the yield value to small flow rate variations or to anyother disturbances generating a variation in the position of theprofile. This high yield sensitivity shows that operation of a unit atits optimum point is difficult manually.

1. A method of optimizing operation of a unit intended for separation ofthe constituents of a feed, comprising a separation loop consisting ofthe interconnection of a series of beds containing a solid adsorbentmaterial forming several zones delimited by feed (F) and solvent (S)injection points and extraction points for discharge, out of the loop,of an extract (Ex) containing a first constituent (Px) of the feed, andof a raffinate (Raf), injection points and extraction points switchingmeans allowing to simulate countercurrent displacement of the beds andmeans for measuring operating variables, comprising using a controlalgorithm for bringing the separation unit to a working point where thepurity of the first constituent in the extract and the yield of theseparation unit as regards production of this first constituent arebrought to specified values, characterized in that, for a given value ofconcentration (Eb_(extract)), in the extract, of a second constituent(Eb) of the feed, the set value of the concentration (Px_(zone1)) of thefirst constituent is adjusted manually in a zone (Z1) located betweenthe solvent injection point and the extract extraction point so as tominimize the solvent/feed ratio (S/F).
 2. A method as claimed in claim1, characterized in that said concentration value (Eb_(extract)) isadjusted so as to maximize the capacity of the separation unit withinthe operating stability limits of said unit.
 3. A method as claimed inclaim 1, characterized in that said set value of concentration(Px_(zone1)) is adjusted by means of a monovariable optimizer.
 4. Amethod as claimed in claim 1, characterized in that the firstconstituent and the second constituent of the feed are paraxylene andethylbenzene respectively.